A color wheel is a wheel used to show the relations of colors. The standard color wheel for light has colors of magenta, yellow, and cyan located at positions that can form an equilateral triangle when connected by straight lines, and another for red, green, and blue. Connecting all 6 of these points forms a hexagon.
Noted that Blue, Yellow, and Red are labeled as the primary while Purple, Green and Orange is labeled as secondary. In presentation graphics, it is the best practice to limit yourself with use of only primary and secondary colors to achieve best contrasting effects. Also, choosing complementary colors (colors that are directly across each other in color wheel) to receive best contrast.
For further information on using color, please consult the following links:
Color Theory
Complementary Color
